Liquid-crystal displays are in extensive use as most important display devices in multimedia societies in applications ranging from cell phones to computer monitors, notebook type personal computers, and TVs. Many optical films are used in a liquid-crystal display for improving display characteristics. In particular, retardation films play major roles such as an improvement in contrast for viewing from the front and oblique directions and color tone compensation. Although retardation films made of polycarbonates and polycycloolefins have been used hitherto, these polymers each are a polymer having positive birefringence. The positiveness or negativeness of birefringence is defined in the following manner.
The optical anisotropy of a polymer film which has undergone molecular orientation by, e.g., stretching can be expressed with the index ellipsoid shown in FIG. 1. In the film which has been stretched, the refractive index in a fast-axis direction of the film plane, the refractive index in an in-plane direction perpendicular to the fast-axis direction, and the refractive index in an out-of-plane vertical direction are expressed by nx, ny, and nz, respectively. Incidentally, the fast-axis is an in-plane axial direction in which the refractive index is low.
Negative birefringence means the case where the stretching direction becomes the fast-axis direction, while positive birefringence means the case where a direction perpendicular to the stretching direction becomes the fast-axis direction.
Namely, the uniaxial stretching of a polymer having negative birefringence results in a reduced refractive index in the stretching axis direction (fast-axis: stretching direction), while the uniaxial stretching of a polymer having positive birefringence results in a reduced refractive index in an axial direction perpendicular to the stretching axis direction (fast-axis: direction perpendicular to stretching direction).
Furthermore, in-plane retardation (Re) is expressed as a value obtained by multiplying the value of [refractive index in an in-plane direction perpendicular to the fast-axis direction (ny)]−[refractive index in a fast-axis direction of the film plane (nx)] by the film thickness.
Many polymers have positive birefringence. Polymers having negative birefringence include acrylic resins and polystyrene. However, acrylic resins are low in the ability to develop retardation, and show insufficient properties when used as an optical compensation film. Polystyrene has: a problem concerning retardation stability that it has a large modulus of photoelasticity in a room temperature region and changes in retardation with a slight stress; a problem concerning optical properties that it has a large wavelength dependence of retardation; and a problem concerning practical use that it has low heat resistance. Presently, polystyrene is not in use.
The term wavelength dependence of retardation means that a retardation changes with measuring wavelength. It can be expressed as the ratio of the retardation as measured at a wavelength of 450 nm (R450) to the retardation as measured at a wavelength of 550 nm (R550), i.e., R450/R550. In general, polymers having an aromatic structure highly tend to have a large value of R450/R550, and use of such polymers results in a decrease in contrast in a short-wavelength region and a decrease in viewing angle characteristics.
A stretched film of a polymer showing negative birefringence has a higher refractive index in the film thickness direction and can be a novel optical compensation film. It is hence useful as an optical compensation film for compensating the viewing angle characteristics of displays such as a super twisted nematic liquid-crystal display (STN-LCD), vertical-alignment liquid-crystal display (VA-LCD), in-plane switching liquid-crystal display (IPS-LCD), reflection type liquid-crystal display, and semi-transmissive liquid-crystal display or as an optical compensation film for compensating the viewing-angle characteristics of polarizers. There is a strong desire on the market for an optical compensation film having negative birefringence. Using a polymer having positive birefringence, processes for producing a film have been proposed to produce a film having a heightened refractive index in the thickness direction. One of these is a method of treatment which comprises bonding a heat-shrinkable film to one or each side of a polymer film and stretching the laminate with heating to apply a shrinkage force in the thickness direction for the polymer film (see, for example, patent documents 1 to 3). Also proposed is a method in which a polymer film is uniaxially stretched in an in-plane direction while applying an electric field thereto (see, for example, patent document 4). Furthermore, a retardation film comprising fine particles having negative optical anisotropy and a transparent polymer has been proposed (see, for example, patent document 5). An optical compensation film or optical compensation layer obtained by applying a liquid-crystalline polymer film and causing the polymer to undergo homeotropic orientation has been proposed (see, for example, patent document 6). An optical compensation film having a coating of an aromatic polymer such as polyvinylnaphthalene or polyvinylbiphenyl has also been proposed (see, for example, patent document 7 and non-patent document 1).
Moreover, an optical film comprising a polyvinylcarbazole type polymer has been proposed (see, for example, patent document 8).
A plastic substrate, optical film, and retardation film for displays have been proposed which comprise a fumaric diester resin or a crosslinked fumaric diester resin (see, for example, patent documents 9 and 10).
[Patent Document 1] Japanese Patent No. 2818983
[Patent Document 2] JP-A-05-297223
[Patent Document 3] JP-A-05-323120
[Patent Document 4] JP-A-06-088909
[Patent Document 5] JP-A-2005-156862
[Patent Document 6] JP-A-2002-333524
[Patent Document 7] JP-A-2006-221116
[Patent Document 8] JP-A-2001-91746
[Patent Document 9] JP-A-2005-97544
[Patent Document 10] JP-A-2006-249318
[Non-Patent Document 1] The Society of Rheology, Japan, vol. 22, No. 2 pp. 129-134 (1994)